PROJECT SUMMARY Developmental dysplasia of the hip (DDH) dramatically increases risk for early development of hip osteoarthritis (OA) in adolescents and young adults. In cases of DDH, abnormal development of the acetabulum (hip socket) and femur alter loading inside the joint, which can lead to painful acetabular labrum tears and articular cartilage lesions. Without correction, this joint damage progresses to OA and may require total joint replacement. Pain and physical limitation are common symptoms in patients with DDH. However, the presentation, intensity and combination of symptoms do not always correspond with radiographic signs of bony abnormalities or soft-tissue damage. Factors beyond bony structure, such as muscle function, likely contribute to symptom onset and altered joint loading, but the role of muscle in DDH pathomechanics is not well understood. This project will be the first to rigorously investigate the bone-muscle relationship as a factor in pathomechanics and symptomatology in patients with DDH. In Aim 1, we will use magnetic resonance imaging (MRI) and musculoskeletal models to compare hip muscle volumes, 3D muscle moment arms, and muscle strength between patients with DDH and control subjects. Aim 1 will provide the first 3D analyses of muscle geometry and moment arms in patients with DDH, and may elucidate relationships among muscle alterations, the severity of bony abnormalities, and muscle weakness. In Aim 2, we will use 3D motion capture to quantify hip movement and loading during activities of increasing biomechanical demand. Analyzing a range of activities that are common for our patients with DDH can identify mechanical distinctions between patients and control subjects that contribute to symptoms and joint damage. In Aim 3, we will identify translational relationships between our laboratory measures and clinical measures of patient reported outcomes (PROs). Identifying such relationships can establish novel links between important realms of patient evaluation ? laboratory measurement and clinical research. Ultimately, we seek coordinated surgical and nonsurgical approaches that optimize joint loading and balance muscle use to improve treatment and delay OA for patients with DDH. Thus, in an exploratory Aim 4, we will compare bone-muscle geometry, strength, hip mechanics, and PROs one year after hip preservation surgery to pre-surgical values and determine the variables predictive of post-surgical outcomes. This proposal builds on the candidate's prior work in intra-articular mechanics and bone morphology and provides new training in MRI, strength testing, modeling, and clinical research with patients with hip disorders. The mentor team consists of physical therapists, orthopedic surgeons and bioengineers who are experts in muscle structure-function, MRI, OA development, and clinical intervention for DDH. Together, the results from this proposal will provide preliminary data for an R01 focused on key factors that affect patient outcomes after surgical and nonsurgical intervention for DDH.